If you want to talk with Korean to practice Korean then kakaotalk(this is an app for your phone). That is what korean use the most. You can do a txt chat and send voice messages and call someone like a phone call for free if you use wifi.

If you want to get a chat very quick with communication WhatsApp is a good chat. If you want to talk using micro and camera Skype is excellent, both chats are free.

Good luck.

I dont think Tango is popular since I know nobody uses it around me. :)

Thanks.  I will check out Kakaotalk.  I am not good enough to text in Korean.  Is Tango popular in Korea?